356
FXUS61 KPHI 092337
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
737 PM EDT Tue Jul 9 2024

.SYNOPSIS...
A weak front positioned across portions of our region should retreat
northward late tonight and Wednesday as a warm front. A cold front
will then arrive later Wednesday night and Thursday as the remnant
low of Beryl tracks northwest of our region. The front remains in
our area Friday and Saturday before dissipating into Sunday. The
next cold front may approach during Tuesday.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
735 PM...The thunderstorms that occurred a little earlier in
portions of our eastern PA and northern NJ zones have weakened
and moved out of the area. Still can`t rule out a stray shower
or storm this evening north of the urban corridor but at this
point think that any additional activity will be extremely
limited as we lose the daytime heating. With the very humid air
mass firmly in place, tonight will be similar to last night with
overnight low temperatures in the 70s across the region, and an
intrusion of marine layer fog for coastal areas. Not sure how
far inland the fog will reach tonight, so have held off on any
land based dense fog advisory.

As for the heat, dewpoints and subsequently heat index values
today were slightly higher than previously expected. With no
dry air advection expected before tomorrow, think we will see
very similar conditions for tomorrow. Consequently, we upgraded the
Heat Advisory along the 95 corridor to an Excessive Heat
Warning with the afternoon update and extended the Heat
Advisory in the southern Poconos through the day tomorrow.

For tomorrow, we`ll also be closely watching the remnant low of
Beryl. The center of the low is still forecast to remain well west
of our region, and interestingly, most deterministic guidance now
shows a very robust dry slot developing on the east side of the low.
That being said, I stayed pretty close to the previous forecast, as
I`m leery of a dry slot developing that quickly. That being said,
storms may stay mostly confined to areas north and west of the fall
line during the day time before we see storms spread any further
eastward. Primary concern with any storms that develop will be heavy
rain due to the very high precipitable water values and deep warm
cloud layer. However, storm motions should be relatively fast which
may limit the flash flood threat somewhat.

For areas generally along and north of the I-78 corridor, also can`t
rule out some severe thunderstorm potential - primarily straight
line winds. The severe risk will be mostly for storms that develop
in the afternoon.

Storms should progress off shore late overnight Wednesday night
leading to at least a brief lull in precipitation.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
The humid conditions lessen Thursday, however it then increases
again Friday although temperatures will turn less hot. Showers and
some thunderstorms increase Thursday night and especially Friday
with some areas of very heavy rainfall and some flash flooding
possible.

As a positive-tilt upper-level through very slowly shifts eastward
across the Great Lakes, adjacent Canada and into the Mid-Mississippi
Valley, a weakening upper-level low turns northwest off the western
Atlantic and makes a run at our area. A front will also be
positioned across our area, which may actually move back westward
some with time.

The above features will make it turn more convectively active
especially on Friday, however Thursday should favor some break in
the high humidity as the plume of 2+ inches of precipitable water
shifts mostly off the coast. Some increase in the surface winds
Thursday should result in some mixing down of the drier air and
therefore dew points are forecast to drop into the 60s for much of
the area. While it will be very warm, the apparent temperatures will
therefore be lower as the dew points drop. Any shower and
thunderstorm activity on Thursday may be confined to the first half
of the day tied to the eastward shifting high precipitable water
plume. A few showers and thunderstorms however cannot be ruled out
the rest of the day given an upper-level trough slowly approaching
from the west. Showers and thunderstorms may then start to increase
from the southeast and south by later Thursday night as the high
precipitable water plums begins to back westward.

As we go through Friday, the plume of 2.0-2.5 inches of precipitable
water is forecast to shift westward as a weaken upper-level low off
the Carolina coasts continues west and northwestward but weakens.
This results in building instability and increasing forcing for
ascent within the high precipitable water axis. Therefore showers
and some thunderstorms are expected to increase through the day. The
freezing levels are forecast to be high and therefore convection
will be efficient rainfall producers. The southerly flow may also
result in some training or backbuilding convection at least for a
time, and if this becomes realized it will enhance the local very
heavy rain/flash flooding risk. Where excessive rainfall rates
occur, local flash flooding will certainly be possible. This looks
to be focused across much of the coastal plain, however this could
also extend farther north and west. Given a moist environment and
increasing coverage of convection, instability should be low enough
and therefore not anticipating a severe thunderstorm risk at this
point. With the expectation of much more cloud cover and increased
convection, temperatures will be noticeably cooler although it will
be very humid. The convection may continue for much of Friday night
before weakening or shifting eastward closer to daybreak
Saturday.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Summary...Some shower and thunderstorm potential, however the heat
looks to build once again especially early next week.

Synoptic Overview...An upper-level trough is forecast to gradually
overspread the Northeast and Mid-Atlantic during the weekend and
right into early next week. This trough may arrive in two pieces
with the second one being stronger than the first. At the surface, a
front is forecast to be stalled near our area Saturday then slowly
shifts south and east or dissipates into early next week.

For Saturday...As an upper-level trough slowly approaches from the
west, a surface front draped across our area should assist in a zone
of convergence. Lingering convection should continue at least in the
morning before some drier air tries to arrive from the west. A plume
of high precipitable water can result in areas of very heavy rain as
the lingering convection should be very efficient rainfall
producers. It will remain very humid for most of the area, however
temperatures are forecast to be low enough to keep heat indices
below advisory criteria.

For Sunday through Tuesday...The upper-level trough is forecast to
overspread the region during this time, and this could be a little
stronger as some shortwave energy amplifies it some. The front in
place may tend to dissipate or shift offshore. Some drying starts to
work in and this should limit convection chances, however still
cannot rule out a diurnally driven shower or thunderstorm especially
across the western zones. Despite the trough, the heat looks to
build again with daytime temperatures getting into the low to mid
90s for many places each day. While dew points may not be quite as
excessive, peak heat indices are currently forecast to reach or
exceed 100 degrees in many areas particularly Monday and Tuesday.
The hot conditions may promote a surface trough in our vicinity
Monday and Tuesday which may focus a shower or thunderstorm, then a
cold front may approach from the northwest later Tuesday.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...Expect VFR for the 4 urban corridor sites with a
better chance of restrictions at RDG, ABE, MIV, and ACY. For
MIV and especially ACY, there is a good potential for at least
some IFR late tonight due to marine fog and stratus. It also
looks like there could be some low stratus developing at RDG and
ABE but confidence is relatively low on this. Southerly winds
generally around 5 knots or less.

Wednesday...Some early morning restrictions as ACY, MIV, RDG,
and ABE as noted above but this should become VFR by around 14z.
After 15Z, there is an increasing chance for showers and storms
to spread into the region from west to east. Most TAF sites
(aside from KABE and KRDG) are unlikely to see TSRA before 18Z,
and may not see much before 00Z. Winds prevailing from the SW
around 10 kt with some gusts to around 20 kt. Moderate
confidence on the overall pattern, but low confidence on timing
of storms.

Outlook...

Thursday and Friday...Periods of sub-VFR probable especially
Thursday night and Friday with an increase in showers and some
thunderstorms.

Saturday and Sunday...Some local sub-VFR conditions possible
associated mostly with a shower and thunderstorm.

&&

.MARINE...
Marine Dense Fog Advisory continues through 18Z Wednesday. We don`t
have any significant changes coming in the wind direction, so the
fog is expected to persist until we start to get rain showers or
stronger winds, which will likely not come until Wednesday afternoon
at the earliest.

Winds and seas should stay below SCA criteria through Wednesday
morning. By Wednesday afternoon though, gusts 25 to 30 KT and seas
approaching 5 ft are expected on the Atlantic Ocean waters. Thus,
have issued a Small Craft Advisory for Wednesday afternoon and
night.

On the Delaware Bay, winds may get close to 25 kt, but confidence is
low, so have held off on issuing an SCA at this time.

Outlook...

Thursday...Southerly winds could be strong enough for some gusts to
25 knots which may build the seas to 5 feet. Since it looks marginal
at this point, held off on a Small Craft Advisory criteria. Showers
and some thunderstorms should be on the increase especially at night
and some storms could produce locally gusty winds.

Friday through Sunday...The conditions should be below Small Craft
Advisory criteria. Showers and some thunderstorms are expected
especially Friday into Saturday with locally gusty winds
possible.


Rip Currents...

Southerly winds will continue to increase through midweek and will
generally be around 15 to 20 mph for both Wednesday and Thursday.
This will result in waves around 2 to 3 feet in the surf zone. Given
the wind component and some higher breaking waves developing, there
is a MODERATE risk for the development of dangerous rip currents
through Thursday for the New Jersey Shore. Due to somewhat lower
waves for the Delaware Beaches, we went with a LOW risk for the
development of dangerous rip currents both days.

For specific beach forecasts, visit weather.gov/beach/phi

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Warning until 8 PM EDT Wednesday for PAZ060-070-
     071-101>106.
     Heat Advisory until 8 PM EDT Wednesday for PAZ054-055-061-062.
     Air Quality Alert until 11 PM EDT this evening for PAZ070-071-
     101>106.
NJ...Excessive Heat Warning until 8 PM EDT Wednesday for NJZ009-010-
     012-013-015-017>020-027.
     Heat Advisory until 8 PM EDT Wednesday for NJZ001-007-008-016-
     021-022.
     Air Quality Alert until 11 PM EDT this evening for NJZ001-
     007>010-012-015-017>019.
DE...Excessive Heat Warning until 8 PM EDT Wednesday for DEZ001.
     Heat Advisory until 8 PM EDT Wednesday for DEZ002-003.
MD...Heat Advisory until 8 PM EDT Wednesday for MDZ012-015-019-020.
MARINE...Dense Fog Advisory until 2 PM EDT Wednesday for ANZ431-450>455.
     Small Craft Advisory from 2 PM Wednesday to 6 AM EDT Thursday
     for ANZ450>455.

&&

$$

SYNOPSIS...Gorse
NEAR TERM...Fitzsimmons/Johnson
SHORT TERM...Gorse
LONG TERM...Gorse
AVIATION...Fitzsimmons/Gorse/Johnson
MARINE...Gorse/Johnson